This disclosure relates, in various embodiments, to electrostatographic imaging members. The imaging members described herein are flexible electrostatographic imaging members which can be used as photosensitive members, photoreceptors or photoconductors useful in electrophotographic systems, including printers, copiers, other reproductive devices, and digital apparatuses. More particularly, the imaging members of this disclosure have an anti-curl back coating (ACBC) comprising two or more layers or sublayers, including an inner layer and an outer layer. The outer layer includes a low surface energy polymer comprising a small amount of siloxane segments in its molecular backbone.
Flexible electrostatographic imaging members are well known in the art. Typical flexible electrostatographic imaging members include, for example: (1) electrophotographic imaging member belts (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove a toner image from a photoreceptor surface and transfer the same image onto a receiving substrate, such as paper. The flexible electrostatographic imaging members may be seamless or seamed belts; seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and ultrasonically welding the overlapped ends together to form a welded seam. Typical electrophotographic imaging member belts include a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anti-curl back coating (ACBC) layer coated onto the opposite side of the substrate layer. An electrographic imaging member belt may, however, have a more simple material structure; it may have a dielectric imaging layer on one side of a supporting substrate and an ACBC layer on the opposite side of the substrate to render flatness. Although the scope of the present disclosure covers the preparation of all types of flexible electrostatographic imaging members, for reasons of simplicity, the discussion hereinafter will focus only on flexible electrophotographic imaging members in a flexible seamed belt configuration.
Electrophotographic imaging members, such as photoreceptors or photoconductors, typically include a photoconductive layer formed on a flexible electrically conductive substrate or formed on layers between the substrate and photoconductive layer. The photoconductive layer is an insulator in the dark, so that during machine imaging processes, electric charges are retained on its surface. Upon exposure to light, the charge is dissipated, and an image can be formed thereon, developed using a developer material, transferred to a copy substrate, and fused thereto to form a copy or print.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990, the disclosure of which is fully incorporated herein by reference, which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer (CTL). Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous CTL and the supporting conductive layer. Alternatively, the CTL may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
In the case where the charge-generating layer (CGL) is sandwiched between the outermost exposed CTL and the electrically conducting layer, the outer surface of the CTL is charged negatively and the conductive layer is charged positively. The CGL then should be capable of generating electron hole pairs when exposed image wise and inject only the holes through the CTL. In the alternate case when the CTL is sandwiched between the CGL and the conductive layer, the outer surface of the CTL is charged positively while the conductive layer is charged negatively and the holes are injected from the CGL to the CTL. The CTL should be able to transport the holes with as little trapping of charge as possible. In a flexible web-like photoreceptor, the electrically conducting layer may be a thin coating of metal on a flexible substrate support layer.
However, as more advanced, higher speed electrophotographic copiers, duplicators and printers have been developed, degradation of image quality has been encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a charge generating layer (CGL), a charge transport layer (CTL) and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoat layer adjacent to another edge of the imaging layers. Such a photoreceptor usually further comprises an anti-curl back coating (ACBC) layer on the side of the substrate opposite the side carrying the conductive layer, support layer, blocking layer, adhesive layer, charge generating layer, charge transport layer and other layers, in order to provide the photoreceptor with the desired flatness.
Typical negatively-charged imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer (CGL), a charge transport layer (CTL). The CTL is usually the last layer to be coated and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 115° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of coated multilayered photoreceptor material is obtained, upward curling of the multilayered photoreceptor can be observed. This upward curling is a consequence of thermal contraction mismatch between the CTL and the substrate support. As the web stock carrying the wet applied CTL is dried at an elevated temperature, dimensional contraction occurs as the solvent evaporates. Because the drying temperature is usually above the glass transition temperature of the CTL, the CTL remains as a viscous solvent and will flow, automatically re-adjusting itself to compensate for the loss of solvent and maintain its dimensions. As the CTL cools down to its Tg, it solidifies and adheres to the CGL. Further cooling of the CTL down to ambient room temperature will then cause the CTL to contract more than the substrate support layer since it has a thermal coefficient of dimensional contraction approximately 3.7 times greater than that of the substrate support. This differential causes tension strain to develop in the CTL; if unrestrained at this point, the imaging member web stock will thereby spontaneously curl upwardly into a 1.5-inch tube. To offset the curling, an anti-curl back coating (ACBC) layer is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, to render the web stock flat.
In this regard, curling of a photoreceptor web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. Although the ACBC layer counters and balances the curl so as to promote flatness, nonetheless typical conventional ACBC formulations, under normal machine functioning conditions, do not always provide satisfactory imaging member belt performance. For example, ACBC wear and electrostatic charging-up are two frequently seen failures which reduce the service life of a belt and require costly belt replacement.
ACBC layer wear also reduces the ACBC layer thickness, causing the imaging member belt to curl upward. Thinning of the ACBC layer results in reduction of its counter-curling force. Curling is undesirable during imaging belt function because different segments of the imaging surface of the belt are then located at different distances from charging devices, causing non-uniform charging and other problems. For example, non-uniform charging distances can manifest as variations in high background deposits during development of electrostatic latent images near the edges of paper.
The ACBC layer is an outermost exposed backing layer and has high surface contact friction when it slides over the machine subsystems of belt support module, such as rollers, stationary belt guiding components, and backer bars, during dynamic belt cyclic function. These mechanical sliding interactions against the belt support module components not only exacerbate ACBC layer wear, they also produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance.
Moreover, high contact friction of the ACBC layer against machine subsystems causes electrostatic charge build-up. This increases the friction and thus requires more torque to pull the belt. In full color machines with 10 pitches the torque can be extremely high due to large number of backer bars used. At times, one has to use two drive rollers rather than just one, which must then be coordinated electronically precisely to keep any possibility of sagging. Static charge build-up in the ACBC has also been found to result in absolute belt stalling, resulting in machine shutdown. In other cases, the electrostatic charge build-up can be so high as to cause sparking and arcing.
Another problem encountered in conventional belt photoreceptors is an audible squeaky sound generated due to high contact friction interaction between the ACBC layer and the backer bars. Moreover, cumulative deposition of ACBC layer wear debris onto the backer bars may give rise to undesirable defect print marks formed on copies because each debris deposit becomes a surface protrusion point on the backer bar and locally forces the imaging member belt upwardly to interfere with the toner image development process. On other occasions, the ACBC layer wear debris accumulation on the backer bars gradually increases the dynamic contact friction between these two interacting surfaces, interfering with the driving motor to a point where the motor eventually stalls and belt cycling prematurely ceases.
One known method of reducing ACBC layer wear is by including organic particles such as polytetrafluoroethylene (PTFE) into the polymer binder to reinforce the ACBC layer. The benefit of this formulation, however, is outweighed by a major drawback in the PTFE particle dispersion stability of the coating solution. PTFE, being two times heavier than the coating solution, forms an unstable dispersion in a polymer coating solution and tends to settle into big agglomerates in the mix tanks if not continuously stirred. The dispersion problem can result in an ACBC with an insufficient, variable, and/or inhomogeneous PTFE dispersion along the length of the coated web, which inadequately reduces friction. Therefore, the production of an ACBC eliminating or minimizing these difficulties, is needed.